Color ink jet printers use either inks prepared with water or other vaporizable solvents or in the alternative, inks which are solid at room temperature and liquid at elevated temperatures. Inks which contain solvents such as water or other vaporizable solvents require that the ink be dried by vaporization of the solvent after the ink has been applied, by the printer, to a substrate such as a sheet of paper and absorbed by the substrate. Inks which are liquid at elevated temperatures will be dried by reducing the elevated temperatures.
A prior problem associated with the color ink jet printing is color bleeding. Color bleeding is the mixing together of dissimilar liquid color inks when printed adjacent to each other on the substrate before the dissimilar color inks are dried and absorbed by the substrate. Typically, a color ink-jet printed graphics or picture is made by printing color print dots adjacent to each other. The adjacent color print dots are in contact with each other. When the adjacent color print dots are in different colors, the color inks of the adjacent color print dots will mix with each other before the color inks are dried or absorbed. This typically causes the boundaries of adjacent color print dots of dissimilar colors to become blurred and not to have sharp clean edges. This is referred to as the color bleeding.
As stated above, color bleeding typically occurs before the dissimilar color inks of the adjacent color print dots are dried and/or absorbed. Typically, the substrate that receives the color inks is a sheet of plain paper or a sheet of specially coated paper. Plain papers are typically composed of a wide variety of copy and bond papers that are used primarily for xerography and typewriting. These plain papers are typically manufactured using a wide variety of wood pulp. This typically causes the plain papers to have widely different pH values, opacities, and surface properties. This typically causes the plain papers to have widely different ink absorption speeds. Therefore, it is typically impossible and impractical to control color bleeding by controlling use of the plain papers.
One prior approach to lessening the color bleeding problem is a checkerboard pattern printing approach. This approach allows the adjacent color print dots to be alternately printed, thus minimizing the color bleeding problem.
By employing the prior checkerboard pattern printing, only every alternate color of print dots are printed during one print pass of the print head. During a subsequent print pass of the print head, the remaining color print dots are filled in. The spacing between the color print dots during each print pass prevents the inks of the color print dots from mixing together and, therefore, reduces the tendency of color bleeding.
One prior disadvantage associated with the checkerboard pattern printing approach is that the throughput of the printer will be substantially reduced if the color printing is accompanied with black text printing which requires optical density (i.e., double printing the black text). When this occurs, four separate print passes of the print head are required to accomplish the combined color and optical density black text printing. FIGS. 1A through 1D illustrate the respective result after each of the four print passes.
Referring to FIGS. 1A-1D, a print pattern 10 is shown that includes six print columns 11-16. Each of print columns 11-16 includes four print dots. For example, print column 11 includes print dots 11a through 11d and print column 16 includes print dots 16a through 16d. Columns 11-14 comprise color print dots and columns 15-16 comprise black text print dots. FIG. 1A illustrates the print result after the first print pass. FIG. 1B illustrates the print result after the second print pass. FIG. 1C illustrates the print results after the third print pass and FIG. 1D illustrates the print result after the fourth print pass.
As can be seen from FIG. 1A, each of print columns 11-16 has alternate print dots printed. For example, column 12 has color print dots 12b and 12d printed and column 15 has black print dots 15a and 15c printed. This is due to the checkerboard filtering function.
Because the black text dots of print pattern 10 need to be double printed, the second print pass will basically deal with double printing the black print dots printed during the first print pass. As can be seen from FIG. 1B, print dots 15a, 15c, 16b, and 16d are double printed with the black color.
Next, the remaining print dots of print pattern 10 are printed, as can be seen from FIG. 1C. At this time, however, the remaining black print dots are not double printed. For example, dots 15b and 15d are not double printed and dots 16a and 16c are not double printed. Those black print dots are then double printed in the fourth print pass, as shown in FIG. 1D.
Therefore, by employing the prior checkerboard pattern printing method, the combined color and optical density black text printing can only be realized in four print passes that greatly reduces the throughput of the printer.